According to a conventional high density data recording system, a converging light beam is projecting upon a recording medium such an optical disk, and desired data are retrieved by digitally processing the reflected light therefrom. According to a certain rewritable optical disk system which may be called magnetic optical disk system, an optical polarization property of each selected point of the optical disk is altered by heating it beyond the Curie point by projecting a laser beam thereon. This is called the Kerr effect. The local differences in the optical polarization property of the optical disk which are produced in this way to store desired data are detected by an optical head to read out the data therefrom.
In such optical disks, a certain eccentricity and a surface waviness are inevitable. An eccentricity of the optical disk is perceived as a tracking error by the optical head which is required to follow a selected one of tracks to read a set of data. A surface waviness is perceived as a focusing error by the optical head which is required to project a sufficiently focused light upon the desired point on the optical disk. Any failure to control the tracking error and the focusing error will result in unfavorable data read-out errors.
There have been proposed a number of methods for achieving a desired focusing control of an optical head, but most of them either required complicated structures and large number of component parts or are only capable of achieving limited error detection sensitivities. Another problem of the prior art focusing control systems is that they have demonstrated substantially nonlinear error detection properties in detecting out-of-focus states and, hence, a desired high-speed control response cannot be attained without creating the problems of control instability.